The present invention relates to processing of data obtained during the drilling of a well borehole. These data are typically produced by sensors positioned in the vicinity of the drill bit and are telemetered to the surface in real time for process for processing or alternatively recorded down hole in real time for subsequent processing and analysis.
In the exploration and production of hydrocarbons, numerous techniques are used to analyze earth formations penetrated by a borehole. Wireline measurements involve a sensor bearing logging tool or sonde which is lowered and raised at a constant predetermined rate within the borehole. Data from the downhole sensors are transmitted to the surface by way of the cable used to raise and lower the logging tool. The resulting "logs" are graphical or digital tabulations of the measured parameters of interest as a function of depth within the well bore and are used to analyze prospective formations for hydrocarbon content. Data in wireline logging operations are typically collected over evenly spaced depth intervals since the final product is a depiction of measured parameters as a function of depth. Collection of data in this manner is controlled by depth indicator signals generated by the system as the speed at which the logging tool is raised or lowered within the well bore. Wireline measurements must be made after the well bore has been drilled. Such measurements can be adversely affected by the build up of mud cake from the drilling fluid and/or other physical changes which occur in the vicinity of the borehole in the time interval between the drilling and logging operation.
There are many advantages in making parametric measurements of interest during the actual drilling operation. The formation is relatively unperturbed and valuable information concerning the petrophysics of the penetrated formations and the physical condition of the borehole can be obtained in real time or near real time. Sensors can indicate in real time when the geological formation of interest has been reached. High pressured formations can be detected at the time of penetration thus permitting steps to be taken to prevent damage to the well bore, drilling equipment and personnel. Measurement while drilling (MWD) is accomplished by placing sensors in the vicinity of the drill bit and either (a) telemetering data to the surface in real time or (b) recording the data downhole to be retrieved and processed intermittently during routine retrieval of the drill string for replacement of the drill bit. Sensors responsive to temperature, pressure, gamma radiation, neutrons, acoustic energy and electromagnetic energy can be used in the MWD system. The primary disadvantage of MWD over wireline measurements lies in the slow rates of data transmission and recording when drilling at fast rates of penetration. When MWD data are recorded in real time, telemetry to the surface is typically via the drilling fluid column since there is no direct electrical connection between the downhole sensors and the surface equipment. Current drilling fluid telemetry rates are of the order of bits per second while wireline telemetry rates over the logging cable are orders of magnitude greater. In addition, MWD data are created during equally spaced time intervals, whether telemetered directly to the surface or recorded downhole for subsequent surface retrieval and processing. To display MWD data as a function of depth and utilize well established depth based signal processing and analysis technology implemented at surface located equipment, MWD data must be accurately transformed from the dynamic time domain to the depth domain. The time to depth domain transformation is not a simple function since the penetration rate of the drill bit is neither constant nor predictable. In addition, transformation can result in gaps in the depth based data resulting from slow MWD data acquisition rates. When drilling at slow rates of penetration, MWD data can be acquired at a much finer axial resolution than wireline data. Existing wireline processing techniques are often inappropriate for MWD data. The present inventive disclosure pertains to new processing techniques which have been developed for the correct conversion of normally evenly spaced time based data to evenly spaced depth based data which maintain the maximum amount of information from the raw data as well as define the quality of the data.
Seismology involves the measurement of reflected or refracted acoustic waves in the time domain and the conversion or "migration" of these data to the depth domain. Acoustic energy is imparted at the surface of the earth and reflections or refractions from subterranean features exhibiting contrasts in acoustic impedance (the product of acoustic velocity and density) are measured at the earth surface in the time domain. Time domain measurements are then converted to depth domain thereby giving the depth of the perturbing subterranean features. Depth conversions are only as accurate as the knowledge of acoustic impedance of all intervening geological strata. U.S. Pat. No. 5,229,940 to Shein S. Wang and David W. Bell teaches a method of generating three dimensional seismic images of earth formations from a series of conventional two dimensional seismic traces or lines. The methods are directed solely to seismic technology wherein no well boreholes are involved. Seismic processing involves the conversion of reflected or refracted acoustic waves measured as a function of time into amplitude as a function of depth using offset velocity measurements and assumptions. The current invention involves the conversion of borehole environs data measured as a function of time into corresponding data as a function of depth using well defined time to depth conversion steps.
The analysis of returned drilling fluid in a well borehole drilling operation is commonly known as "mud" logging. The returned drilling fluid is analyzed for traces of hydrocarbons including gas, and the returned drill cuttings are analyzed to determine the porosity, lithology and permeability of the strata being penetrated by the drill bit. Fluid and cuttings are analyzed at the surface of the earth and must be assigned a depth of origin in order to correlate with conventional wireline logs, MWD logs and seismic data. U.S. Pat. No. 5,237,539 to Thomas H. Selman teaches methods of determining the "lag" time for the returned drilling fluid to travel from the drill bit to the surface. This, when correlated with the known depth of the drill bit, allows one to assign a depth of origin corresponding to each volumetric increment of returned drilling fluid. The subject patent also teaches depth correlation of lag time parameters with parameters measured in real time including drill bit depth, rate of penetration, weight on bit drill string revolutions per minute and drilling fluid pump pressure. Real time and lag time parameters plotted as a function of depth constitute a conventional "mud" log. All of the teachings of the subject patent involve measurements at the surface wherein no downhole sensors are employed. All data are acquired during equal depth intervals rather than during equal time intervals. The patent also teaches depth correlation of real and lag time parameters using a simple linear depth shift of the data. There is no teaching of quality indicators reflecting the accuracy or reliability of the lag time conversions.